In order to couple a vehicle together with a towed vehicle, use is made of a draw coupling. The draw coupling has a coupling mouth which facilitates introduction of the towing eye into the coupling jaw of the draw coupling. The draw coupling comprises a lifting mechanism where the moving parts are located in a protected manner. A coupling bolt which can be raised and lowered by a handle is lowered through the towing eye when the towing eye is correctly positioned. Lowering of the coupling bolt can often take place automatically when the towing eye is correctly positioned in the draw coupling. This is affected, for example, using a device which releases the coupling bolt when the towing eye is correctly positioned in the draw coupling. The coupling bolt can also be pneumatically controlled, that is to say the raising and lowering are effected using compressed air.
The draw coupling is fixed in the rear crossbar of the vehicle. The towing arrangement is often provided with a mechanism which makes it possible for the towing arrangement to rotate around its center axis if the trailer should tip over. The draw coupling can also comprise a signal pin which shows that the coupling bolt has been lowered into the correct position.
The draw coupling is dimensioned for the load for which it is intended, for example for the total weight of the towed vehicle. The legal requirements of different markets also affect the dimensioning of the draw coupling. The towing section of the coupling bolt, that is to say the portion which bears against the towing eye, is designed as a slightly ball-shaped portion so as better to take up the loads which arise when the vehicle and the towed vehicle move in relation to one another. An example of a known draw coupling can be found in, for example, EP 1015262.
A problem with the draw couplings found on the market is that a draw coupling is intended for only one size of towing eye. On the market, there are a number of different standards for towing eyes and thus also for draw couplings. The most common are towing eyes with diameters of 40 mm, 50 mm and 57 mm. 40 mm is an older, European standard which is being replaced by 50 mm. 57 mm is used mainly in northern Europe. The height of the towing eyes can also vary between different types; this means that the coupling mouth of the towing arrangement is matched to one type of towing eye and that when a vehicle is to tow a towed vehicle, the driver must first check that the dimensions of the towing bolt and the towing eye are compatible. If this is not the case, either the draw coupling has to be changed, which is extremely complicated, or the towing eye on the drawbar of the towed vehicle has to be changed, which is also complicated. Moreover, the towed vehicle is type-approved together with the towing eye and drawbar, which means that it may not be permitted to change the towing eye.
The problem with differently dimensioned towing eyes renders it impossible in principle for a vehicle to be coupled to a towed vehicle which has a towing eye of a different size.
This may occur, for example, when a northern European vehicle is to tow a towed vehicle from central Europe. As the transport market is becoming increasingly deregulated and there are consequently vehicles registered in different countries on the same market, this is a growing problem.